Authorities have been able to conclusively determine that the horrific attack in Manchester was, in fact, a terrorist attack. After an explosion went off at an Ariana Grande concert, 22 people were killed and dozens more were injured. And now, ISIS has officially claimed responsibility.

Tuesday, ISIS released a statement on the encrypted messaging app “Telegram.” “One of the soldiers of the Caliphate placed explosive devices in a gathering of crusaders in the middle of the British city of Manchester,” the statement said.

21,000 people were present at the concert when the bomb went off. The concert had just ended, with concert-goers, many of whom were children, filing out of the building. ISIS sympathizers immediately began celebrating online, with one Twitter user posting a video in which he said, “This is only the beginning. The lions of Islamic State … are beginning to attack all the crusaders.”

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Dashcam video from outside of the arena captured the explosion, in footage that has quickly gone viral. In it, a faint flash can be seen and then a loud boom can be heard. The man in the video shouts, “No!,” seemingly understanding what has just happened.

Grande herself was not injured. She had just left the stage when the explosion happened. Concert-goers had remained behind in the arena as the lights went up and they were playing with pink balloons that had been released. Many initially believed that the boom was balloons popping, until they realized what had happened and panic and hysteria broke as people began screaming and running for the exits.

For her part, Grande responded on Twitter, expressing her heartbreak and sorrow at what had happened.

broken. from the bottom of my heart, i am so so sorry. i don't have words.